[llvm] [LoopRotate] Set loop back edge weight to not less than exit weight (PR #86496)

[Haohai Wen via llvm-commits]

================
@@ -347,9 +347,19 @@ static void updateBranchWeights(BranchInst &PreHeaderBI, BranchInst &LoopBI,
         // probabilities as if there are only 0-trip and 1-trip cases.
         ExitWeight0 = OrigLoopExitWeight - OrigLoopBackedgeWeight;
       }
+    } else {
+      if (OrigLoopExitWeight > OrigLoopBackedgeWeight) {
+        LLVM_DEBUG(
+            dbgs() << "WARNING: Bad loop back edge weight. Adjust it from "
+                   << OrigLoopBackedgeWeight << " to " << OrigLoopExitWeight
+                   << "\n");
----------------
HaohaiWen wrote:

The heuristic wasn't based on guessing. I think those three expressions were bullet proofed (@MatzeB , please correct me if I am wrong). It just have a pre request the profile data is accurate. What I'm trying is to fix inaccurate profile data.
```
  //  -  x == x0 + x1        # counts to "exit" must stay the same.
  //  - y0 == x - x0 == x1   # how often loop was entered at all.
  //  - y1 == y - y0         # How often loop was repeated (after first iter.).
```

https://github.com/llvm/llvm-project/pull/86496